Hi Sarah,
Hmmm....is it possible that these students have a pop-up blocker installed?
Could they try and open the document in an incognito window and see if they then can see the rubric?
This next step might be hard but if you could help one of the students load the error console when they open the document (the error console could tell us if any errors are occurring on the user-facing side of the add-on) To do this: hold down Control+Shift+J" on a PC or "Command+Option+I" on a Mac and when the side window opens, take a screenshot of the error console ( make sure the side window is on the 'console' tab)
Thanks!!